Some seedlings not looking great

[wxcrawler]

Hi everyone,

Hopefully my attached picture is viewable. These are 2 Sun Sugar plants that were germinated on Feb 15. They are an example of what's going on with about half of my 30 seedlings (9 different varieties). As you can see, I'm getting major whitening of the leaves. They haven't been outside, so it's not sunburn. They are growing under T5 6500K lights, 16 hours on and 8 hours off. They are in cow pots. My mix is a combination of 60% Roots Organics CoCo Coir, 30% Diatomaceous Earth (the new Floor Dry from AutoZone), and 10% Fox Farms Ocean Forest. When I wetted the Coir, I added 1/4 strength Texas Tomato Food Apples and Oranges to 3 of the 5 gallons of water used for wetting. They get bottom-watered every 4 days with distilled water (when needed). They are in a room that stays around 68 degrees, and they have a small fan lightly blowing on them when the lights are on.

What's weird to me is this is only occurring on about half of the seedlings, and it is not variety-dependent. I have 2 other Sun Sugars that show no signs of this.

Any guesses on what the could be the problem?

Any insight would be greatly appreciated.

Thanks.

Lee